With the recent reduction in size and weight of electronic devices, there is an increasing demand for batteries with high energy densities. This has lead to active research and development of lithium primary batteries that use lithium metal as a negative electrode active material and lithium ion secondary batteries that use carbon material as a negative electrode active material.
Such lithium batteries that use lithium metal or carbon material as negative electrode active materials use, for example, an organic electrolyte including an organic solvent and a solute dissolved therein. Typical organic solvents used for forming organic electrolytes include propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, sulfolane, dimethoxyethane, tetrahydrofuran, dioxolane, and γ-butyrolactone. They are used singly or in combination with two or more of them. Also, exemplary solutes include LiClO4, LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, and LiN(CF3SO2)(C4F9SO2).
Recently, extensive studies have also been conducted on lithium polymer batteries using a gel electrolyte composed of a combination of an organic electrolyte and a polymer and all-solid lithium polymer batteries using a polymer solid electrolyte.
Exemplary polymers used for forming gel electrolytes are polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyacrylonitrile (PAN), poly(methyl methacrylate) (PMMA), and polymer-based derivatives such as polysiloxane.
For gel electrolytes and solid polymer electrolytes, essentially the same solutes as those used in organic electrolytes are used.
It is known that substances forming organic electrolytes chemically react with moisture inside a battery, a positive electrode or a negative electrode. Particularly, organic electrolytes are highly reactive with respect to lithium metal, lithium alloy (e.g., Li—Al, Li—Sn), and lithium-containing carbon material, which are negative electrode active materials. Due to the chemical reaction between a negative electrode and an organic electrolyte, for example, a film made of decomposition products of the organic solvent is formed on the surface of the negative electrode, thereby resulting in an increase in internal resistance of the battery. Therefore, if the battery is stored for an extended period of time, the increase in internal resistance of the battery causes a large voltage drop upon discharge, which may lead to insufficient discharge characteristics.
In secondary batteries, repetitive charge/discharge cycling also increases the internal resistance of the battery, resulting in degradation in cycle characteristics.
As such, for example, Japanese Laid-Open Patent Publication No. Hei 7-22069 proposes adding an additive that forms a stable film on the negative electrode surface to an organic electrolyte, in order to suppress the increase in internal resistance of an organic electrolyte battery. As such an additive, for example, an aromatic dicarboxylic acid ester is used.
However, if such an additive is added to an organic electrolyte, the film formed on the negative electrode surface has a relatively large resistance. Hence, it is difficult to obtain sufficient discharge characteristics.
It is therefore an object of the present invention to provide an organic electrolyte battery in which the increase in internal resistance is suppressed upon storage. It is another object of the present invention to provide an organic electrolyte battery with improved charge/discharge cycle characteristics.